The present invention relates to a method and apparatus for determining the gel time of thermoset resins at a particular reaction temperature. The gel time is the length of time it takes for the thermoset to begin to "gel" or harden.
One common prior art method for determining thermoset gel time involves placing the resin on a hot plate, stirring it until it becomes sticky and measuring the passage of time to that point. This is a rather imprecise method, but it is fairly popular because no expensive equipment is required to practice the method.
A much more precise method for measuring thermoset resin gel time involves the use of a very expensive piece of equipment known as a dynamic mechanical spectrometer (DMS) sold by Rheometrics. In accordance with this method, the thermoset resin is placed in a heated cup, and a plate or cone is lowered into the resin sample. The plate or cone is subjected to a vibratory spin, and the viscosity of the resin as measured by resistance to vibratory spin on an electric motor, is determined. The point at which the viscosity begins to rise quickly is the gel point, which establishes the gel time.
While this method and equipment yields accurate and reproducible results, the equipment is capital intensive. Further, the method must be finished before the thermoset cure is complete. The cup and the plate or cone must be cleaned before the cure is complete. Otherwise, the resin will have hardened in the cup and on the plate or cone, making it extremely difficult, and possibly impossible to clean.